### Re: How can an ariplane fly upside down?

Area: Engineering
Posted By: William Beaty, Engineer / Interactive exhibit designer
Date: Sat Aug 2 14:29:45 1997
Area of science: Engineering
ID: 867681964.Eg
Message:

Hi Matt!
I suspect that you might be actually asking this: since an airplane's wing must be longer and more curved on top in order to create aerodynamic lift, how can a plane still fly when it is upside-down? (Am I right? If not, then I'm not answering your question!)

Many textbooks claim that airplane wings create lift like so:

• The upper surface of the wing is longer and more curved than the lower surface.
• Air is divided by the leading edge of the wing, and the air must join together again after it passes the trailing edge of the wing.
• Because the upper surface of the wing is longer than the lower surface, the air must flow faster over the upper surface, otherwise it wouldn't rejoin at the trailing edge..
• Because of the Bernoulli Effect, faster moving air exerts less pressure than slow air.
• The fast air above the wing sucks the wing upwards, creating lifting force

Unfortunately this line of reasoning is wrong. And worse, it widely "infects" textbooks of all kinds. See K-6 Science Textbook Misconceptions under wing. Also see Airfoil Lifting Force Misconception.

The above "Bernoilli" explanation is wrong in several places. Most importantly, wind-tunnel photographs prove that the air which divides at the leading edge of the wing does not rejoin at the trailing edge. Other problems: thin wings such as Hang Gliders, sailboat sails, paper airplanes, and even the Wright Brothers' flyer all have equal pathlengths above and below the wing, yet these craft can fly. Also, as you point out, inverted flight is not as impossible as the "Bernoulli" explanation would have you believe. Here is an alternative explanation:

• Aircraft wings are tilted (called "angle of attack").
• As they cut through the air, they force air downwards (they even force the air flowing above them to move downwards.)
• For every acting force, there is a reacting force, so as the wing forces the air downwards, the air forces the wing upwards.
• Because of "conservation of momentum", the wing can only fight against gravity if it throws air downwards.
• The plane flys, it essentially "surfboards" across the air.

So, how can a plane fly upside-down? The pilot simply flips the plane (or performs half of a loop-the-loop), then uses the control surfaces ("flaps") to tilt the plane so the wing still forces air downwards. But this is what the pilot was doing for right-side-up flight too: tilting the plane so the wing acts downwards upon the air.

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